This is a guess but you might have a mismatch between your versions of appium, java-client, and selenium-java…
Try upgrading your appium version to 1.8 and then upgrade the corresponding libraries…
I have:
appium = 1.8
io.appium.java-client = 6.0.0
org.seleniumhq.selenium.selenium-java = 3.9.1
JDK = 1.8